add proxy cookie leak repro test

[MustafaMulla29]

Summary

• add a local-only integration test for the CLI dev server's /api/proxy route
• send a synthetic PostHog-style localhost cookie through the proxy
• verify that the upstream target does not receive proxy-origin cookies
• avoid any dependency on EasyEDA or other external services

Why

A standard USB-C connector can fail to load in tsci dev because the browser sends its localhost analytics cookie to /api/proxy. The proxy currently forwards that cookie to EasyEDA when X-Sender-Cookie is empty, and EasyEDA's CloudFront/WAF rejects the request with 403 Forbidden.

The test reproduces the header leak directly using local CLI and target HTTP servers.

Current result

This draft intentionally contains a failing regression test until the proxy fix is implemented:

expect(received).toBeUndefined()
Received: "ph_test_posthog=..."

Validation

• bun run format:check — passes
• bun test tests/cli/dev/proxy-cookie-isolation.test.ts — fails as expected, proving the synthetic localhost cookie reaches the target